The present invention relates generally to ice prevention systems for aircraft structures. The problems relating to the formation of ice on aircraft structures are well known. In certain climatic conditions water droplets may exist at subfreezing temperatures in a liquid state. These supercooled droplets nucleate and form ice upon contact with the aircraft surfaces. Ice therefore tends to form on the leading edges of aircraft structures. The ice can degrade aircraft performance through increasing the effective weight of the aircraft and by increasing drag resistance and reducing lift provided by the airfoils. In the case of propeller and rotor icing, asymmetries in the ice build-up can cause serious vibrations resulting in structural damage.
Various techniques have been employed in the past in an attempt to avoid the problems caused by icing. These include deicing devices which remove the ice by scraping or cracking, or which melt the ice with microwave heating. One problem with such prior art deicing devices is that the ice is removed in large pieces which then become projectiles threatening to the aircraft. Another problem is that propellers and rotors can deice asymmetrically causing the difficulties described above. Other prior art devices employ electrical heating elements within the structure as deicing or anti-icing devices. Electrothermal devices are by nature inefficient for two reasons. first they melt ice which requires substantially more energy than simply heating the ice to above freezing due to the latent heat of fusion required to melt ice of 334 joules/gm. Microwave deicing devices are also susceptible to this inefficiency. Second, electrothermal devices heat large areas of the airfoil which are then cooled as conduction carries off heat to both the cooler parts of the structure and to the environment with only a small fraction of the energy actually melting ice. These inefficiences are a major problem in aircraft systems where the total available power is, in general, quite limited.
It will be apparent from the foregoing that there is a clear need for an ice prevention technique which avoids or minimizes the disadvantages of the prior art. The present invention fulfills this need.
It will also be apparent from the following that although the present invention is described in terms of aircraft structures, it will work equally well on any surface in any environment where icing is a problem.